Methods of forming a polymer layer on a polymer surface

ABSTRACT

Methods of forming polymer layers on polymer surfaces using surface initiated atom-transfer radical-polymerization (ATRP) are described. The method can include functionalization steps prior to performing surface initiated ATRP, such as hydroxylation steps and/or halogenation steps. The hydroxylation step can be carried out in a solution including potassium persulfate, ammonium persulfate, or lithium hydroxide. The halogenation step can also be carried out in a solution. The methods described herein can be performed on bundles of hollow polymer fibers, including bundles of hollow polymer fibers mounted in a module.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation application of U.S. patentapplication Ser. No. 14/223,583 filed Mar. 24, 2014, which claimspriority to U.S. Provisional Application No. 61/826,141, filed May 22,2013, the entirety of which is hereby incorporated by reference.

BACKGROUND

Membranes are widely used in a variety of applications to separate andpurify liquid and gas streams. The most widely used material for theseparation layer of membranes are polymers. Many polymeric materialshave desirable membrane properties. However, many of these polymermaterials can only be used by forming the polymer material on a supportmaterial. Exemplary support materials include polymers, plastics,metals, ceramics, and organic material. The support material may beporous or non-porous. The support material is generally required becausethe separation layer is thin and delicate.

Various methods have been used to form thin separation layers on supportmaterial. Traditional approaches involve solution-deposition, plasmapolymerization, interfacial polymerization, and doctor blade approacheson flat sheets. These techniques may have various drawbacks, such aslimited choice of applicable materials, high expense, and lack ofcoating durability.

Another method that has been used to form thin polymer separation layerson the surface of support materials is surface initiated radicalpolymerization. However, various issues arise with such techniques. Forexample, a free radical polymerization process initiated from a surfaceis generally not controllable. As a result, the polymer layer formed bythis method may be too thick to function and/or may lack uniformthickness.

Controlled radical polymerizations initiated from a surface have meetwith greater success. The slow and controlled nature of these reactionsallows for uniform coatings to be formed. Unfortunately, previouslyavailable techniques of controlled radical polymerization involve theuse of harsh conditions and/or environments that were not economicallyviable to scale.

Atom-transfer radical-polymerization, such as disclosed in U.S. Pat.Nos. 5,763,548 and 5,789,487, allows for controlled radicalpolymerization reactions to be performed using inexpensive reagents inmild conditions. Varieties of ATRP may be performed at room temperatureand pressure with aqueous solvents.

The process of carrying out surface initiated atom-transferradical-polymerization generally requires one or more preparation stepsin which the surface on which a layer of material is formed isfunctionalized. Functionalization steps can include hydroxylating thesurface and halogenating the surface. Various drawbacks associated withthese functionalization steps have contributed to surface initiated atomtransfer radical-polymerization not being usable on a commercial scale.

In the traditional hydroxylation step, gases such as ozone have beenused to add —OH groups to the surface of a material. The use of ozonecomplicates the overall process because it can rapidly degrade themechanical properties of the polymers upon exposure.

In the traditional halogenation step, acyl halides such asbromoisobutyryl bromide has been used. The use of such acyl halidesposes difficulties due to the air and water sensitivity of thesereagents (i.e., they environment must be kept air and water free). As aresult, scale up of the halogenation process is difficult or impossible.

The problems identified above with functionalization have also posedproblems with respect to scale up. To date, no methods ofsurface-initiated atom-transfer radical polymerization has beendeveloped which can be used to add polymer coatings to surfaces at highvolumes and with relatively low costs.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the methods and systemsdisclosed herein are described with reference to the following figures,wherein like reference numeral refer to like parts throughout thevarious views unless otherwise specified.

FIG. 1 is a flow chart illustrating a method of forming a polymer layeron a polymer surface according to various embodiments described herein;

FIG. 2A is a schematic view of a hydroxylation process carried out on apolymer surface according to various embodiments described herein;

FIG. 2B is a schematic view of a halogenation process carried out on asurface according to various embodiments described herein;

FIG. 2C is a schematic view of an ATRP process carried out on a polymersurface according to various embodiments described herein;

FIG. 3 is a cross sectional view of a module suitable for use variousembodiments of methods described herein;

FIG. 4 is a schematic view of a module suitable for use in variousembodiments of methods described herein;

FIG. 5 is a schematic view of a module suitable for use in variousembodiments of methods described herein; and

FIG. 6 is a schematic view of a reel to reel system suitable for usewith various embodiments of the methods described herein.

DETAILED DESCRIPTION

Overview

The instant disclosure is directed to various methods of forming thinnon-porous polymer layers on porous or non-porous polymer surfaces,wherein the polymer layers are formed using surface initiatedatom-transfer radical polymerization. In some embodiments, the methodsare related to performing these methods on bundles of hollow polymerfibers in an effort to produce materials well suited for variousseparation applications. The method can include the use of varioussolutions formulated to carry out functionalization steps and/oratom-transfer radical polymerization steps. In some embodiments, thefunctionalization steps can include the hydroxylation and halogenationof the polymer surface. These functionalization steps prepare thepolymer surface for atom-transfer radical polymerization. The variousmethods described herein can also be performed in uniquely designedmodules. The modules allow the polymer surface to be mounted within aclosed vessel. Once mounted, various solutions can be passed into andout of the module in order to carry out the methods described herein. Insome embodiments, the modules are configured in a shell-and-tube typeconfiguration, which allows for selective exposure of the polymersurfaces to the solutions passed through the module (e.g., only anexterior or interior surface is exposed to the solution).

References throughout this specification to “one example,” “an example,”“one embodiment” or “an embodiment” mean that a particular feature,structure, process or characteristic described in connection with theexample is included in at least one example of the present technology.Thus, the occurrences of the phrases “in one example,” “in an example,”“one embodiment” or “an embodiment” in various places throughout thisspecification are not necessarily all referring to the same example.Furthermore, the particular features, structures, routines, steps orcharacteristics may be combined in any suitable manner in one or moreexamples of the technology. The headings provided herein are forconvenience only and are not intended to limit or interpret the scope ormeaning of the claimed technology.

Certain embodiments of the technology described below may take the formof computer-executable instructions, including routines executed by aprogrammable computer or controller. Those skilled in the relevant artwill appreciate that the technology can be practiced on computer orcontroller systems other than those shown and described below. Thetechnology can be embodied in a special-purpose computer, controller, ordata processor that is specifically programmed, configured orconstructed to perform one or more of the computer-executableinstructions described below. Accordingly, the terms “computer” and“controller” as generally used herein refer to any data processor andcan include internet appliances, hand-held devices, multi-processorsystems, programmable consumer electronics, network computers,mini-computers, and the like. The technology can also be practiced indistributed environments where tasks or modules are performed by remoteprocessing devices that are linked through a communications network.Aspects of the technology described below may be stored or distributedon computer-readable media, including magnetic or optically readable orremovable computer discs as well as media distributed electronicallyover networks. In particular embodiments, data structures andtransmissions of data particular to aspects of the technology are alsoencompassed within the scope of the present technology. The presenttechnology encompasses both methods of programming computer-readablemedia to perform particular steps, as well as executing the steps.

In some embodiments, a method of forming a polymer film on a polymersurface in which a hydroxylation solution is used is disclosed. Themethod can include a step of hydroxylating a polymer surface in ahydroxylation solution. The hydroxylation solution can include, forexample, potassium persulfate, ammonium persulfate, or lithiumhydroxide. The method can also include a step of halogenating thepolymer surface. The method can also include a step of performingsurface initiated atom-transfer radical-polymerization on the polymersurface. Performing surface initiated atom-transfer radicalpolymerization forms a polymer film on the polymer surface.

In some embodiments, a method of forming a polymer film on a polymersurface in which a halogenation solution is used is disclosed. Themethod can include a step of providing a polymer surface having hydroxylgroups bonded to the polymer surface. The method can further include astep of halogenating the polymer surface in a halogenation solution. Insome embodiments, the halogenation solution includes hydrobromic acid,hydrochloric acid, phosphorous tribromide, phosphorous trichloride,hydroiodic acid, thionyl chloride, thionyl bromide, or combinationsthereof. The method can also include a step of performing surfaceinitiated atom-transfer radical-polymerization on the polymer surface.The surface initiated atom-transfer radical-polymerization step canresult in the formation of a polymer film on the polymer surface.

In some embodiments, a method of forming a polymer layer on a bundle ofhollow polymer fibers is disclosed. The method can include a step ofproviding a bundle of hollow polymer fibers. The method can furtherinclude a step of performing surface initiated atom-transferradical-polymerization on the bundle of hollow polymer fibers. Thesurface initiated atom-transfer radical-polymerization step can resultin the formation of a polymer film on the polymer surface.

Polymer Layer Formation Method

With reference to FIG. 1, a method of forming a polymer layer on polymersurface generally includes a step 110 of hydroxylating the polymersurface, a step 120 of halogenating the surface, and a step 130 ofperforming surface initiated atom-transfer radical-polymerization on thepolymer surface to thereby form a polymer layer on the polymer surface.

In step 110, a polymer surface is provided and subjected to ahydroxylation process in order to add a plurality of hydroxyl groups tothe polymer surface. Addition of the hydroxyl groups is generally aimedat providing a mechanism for bonding a halogen atom to the polymersurface.

The polymer surface upon which the polymer layer is formed can generallybe a surface of any hydrocarbon-containing polymer material. In someembodiments, the polymer surface is selected from a range of materialsknown to be suitable for separation processes, such as in the sweeteningof natural gas. Exemplary polymer materials suitable for use in step 110include, but are not limited to, polymethyl pentene, polypropylene,cellulose acetate, polyimide, polyvinylidene fluoride (PVDF), andderivatives thereof. The polymer surface provided in step 110 can be aselectively permeable material or a material that is not selectivelypermeable.

In alternate embodiments, the surface upon which the polymer layer isformed can be a non-polymer material. For example, the surface can bethe surface of a ceramic, metal, organic, or plastic material. Thesematerials may be selectively permeable or may not be selectivelypermeable.

The polymer surface can be part of a variety of different forms. In someembodiments, the polymer surface is a surface of a polymer fiber. Whenpolymer fibers are used, the methods described herein can be carried outon isolated polymer fibers or bundles of polymer fibers (includingbundles of fibers that are potted together). When bundles of polymerfibers are used, the polymer fibers are generally aligned in paralleland arranged (or cut) to have coplanar terminal ends. The polymer fibersused in the methods described herein can also be hollow. When hollowfibers are used, the surface upon which the ATRP step is performed canbe an interior and/or an exterior surface. In other embodiments, thepolymer surface is the surface of a flat sheet, such as a spiral woundsheet.

The dimensions of the material providing the surface upon with ATRP iscarried out are generally not limited. When polymer fibers are used, thelength and diameter of the individual fibers are generally not limited.Similarly, when bundles of fibers are used, there is generally no limitto the number of fibers in a bundle, the overall length of the bundle orthe overall diameter of the bundle. When spiral wound sheets are used,the length, width, and thickness of the sheets are generally notlimited.

The material providing the surface upon which the ATRP step is performedis can be a porous or non-porous material. In some embodiments, thematerial has selectivities relative to certain compounds. The specificporosity and selectivities of the material is generally not limited. Insome embodiments, the porosity of the material is generally within therange of from less than 1% to 90%. The size of the individual pores inthe material can be in the range of 300 nm or smaller. The porosity ofthe material is generally consistent throughout the material.

In some embodiments, the polymer surface can be subjected to a cleaningstep prior to performing step 110. The cleaning step is generally aimedat removing residual oil or other debris from the surface of thematerial. Any cleaning steps capable of removing unwanted material fromthe surface of the material can be used. In some embodiments, thepolymer surface is immersed in or rinsed with a solution capable ofremoving unwanted material from the surface of the polymer material. Insome embodiments, the solution includes acetone, methanol, organicsolvents, or any combinations thereof. When the polymer surface isimmersed in the cleaning solution, the cleaning process can includesequential baths in order to improve the amount of material removed.When the polymer surface is rinsed with the cleaning solution, therinsing can be continuous and for any period of time to remove theunwanted material. The cleaning step can also be carried out in avariety of apparatus, including in an soxhlet extraction apparatus. Thecleaning steps described above can be performed on any form of thepolymer surface, including on isolated polymer fibers, on bundledpolymer fibers, and on potted bundles of polymer fibers.

In step 110, the polymer surface is hydroxylated in order to add one ormore hydroxyl groups to the surface of the material. Hydroxylation asused herein refers to the addition of one or more —OH groups to thepolymer surface via covalent bonding between the polymer membranematerial and the hydroxyl group. FIG. 2A illustrates the hydroxylationstep, with a plurality of —OH groups 210 bonded to the surface of thematerial 200. The hydroxyl group 210 can bond to the polymer surface 200via any suitable atom of the material. In some embodiments, thehydroxylation step removes a hydrogen atom from the polymer surface toprovide a bonding cite between the polymer surface and the oxygen atomof the hydroxyl group. As discussed further below, the hydroxyl groupsbonded to the polymer surface provide a mechanism for subsequentlybonding a halogen atom to the polymer surface.

Any suitable method of hydroxylating the polymer surface material can beused. In some embodiments, the polymer surface is subjected to ahydroxylation solution which results in the addition of the —OH groupsto the polymer surface. The hydroxylation solution can include any of avariety of suitable solutions. In some embodiments, the hydroxylationsolution is a solution that includes potassium persulfate, ammoniumpersulfate, or lithium hydroxide, or combinations thereof. In someembodiments, the solution is an aqueous solution and therefor includeswater.

In one embodiment wherein potassium persulfate or ammonium persulfate isused to perform hydroxylation, the hydroxylation step 110 is carried outby first preparing a bath of deoxygenated water which is continuouslyheated at 80° C. or higher. The potassium persulfate and/or the ammoniumpersulfate is then added to the water without introducing oxygen. Thepotassium persulfate and/or the ammonium persulfate can also be addedprior to heating the deoxygenated water (i.e., the potassium persulfateand/or ammonium persulfate is added to the deoxygenated water, followedby heating of the bath to 80° C. or higher). The polymer surface canthen be immersed in a bath of the solution or, when the polymer surfaceis mounted in a module, the solution can be flowed through the module.Generally speaking, the polymer surface is then exposed to the solutionfor a period of time (e.g., 10 minutes or more), during whichhydroxylation of the surface of the material occurs. After the desiredperiod of time has passed, the polymer surface is removed from thesolution and rinsed.

In embodiments where lithium hydroxide is used, the polymer surface isgenerally a fluorinated polymer material. A specific fluorinated polymerthat can be hydroxylated using lithium hydroxide is PVDF.

Other suitable hydroxylation solutions includes, ceric ammonium nitrate,ozone gas, piranha solutions (e.g., Caro's acid, peroxymonosulfuricacid, etc.), Fenton type reagents (e.g., hydrogen peroxide, ironsulfate, etc.), and chromate solutions. These hydroxylation materialscan be used to carry out the hydroxylation step 110 in a similar oridentical fashion to the method described above. In one embodiment,hydroxylation is carried out using potassium persulfate with ironsulfate in a Fenton-type reaction.

Other suitable hydroxylation methods can be used. For example,hydroxylation can be carried out without the use of a solution. In oneexample, the polymer surface can be oxidized using plasma etching,exposure to arc, or by exposure to flame.

In some embodiments, the polymer surface can be obtained from a thirdparty with the surface having already been hydroxylated. Similarly, thematerial used in the methods described herein can naturally havehydroxyl groups available on the surface of the material, in which casethe hydroxylation step 110 need not be carried out.

Once the hydroxylation step 110 has taken place, a halogenation step 120can be performed. The halogenation step 120 is generally aimed atbonding a halogen atom to the polymer surface. This can be accomplishedeither by bonding the halogen atom with the oxygen atom of the hydroxylgroup or by replacing the hydroxyl group with the halogen atom (i.e.,providing direct bonding between the halogen atom and the polymersurface). Halogenation as used herein refers to the addition of one ormore halogen atoms to the polymer surface via covalent bonding betweenthe hydroxyl group and the halogen atom or between the halogen atom anda carbon atom of the polymer surface (i.e., by completing removing thehydroxyl group). FIG. 2B illustrates the halogenation step, with aplurality of halogen atoms (HA) 220 bonded to the polymer surface 200via the hydroxyl group 210 or a plurality of halogen atoms 220 bondingdirectly to the polymer surface 200 by replacing the hydroxyl group. Insome embodiments, the halogenation step generally involves theattachment of a molecule having a radically transferable group (i.e., ahalogen) to the hydroxyl group. Bonding the radically transferable group(e.g., the halogen atom) to the polymer surface sets the stage for thesubsequent performance of ATRP on the polymer surface as discussed ingreater detail below.

Any suitable method of halogenating the polymer surface material can beused. In some embodiments, the polymer surface is subjected to ahalogenation solution which results in the addition of the halogen atomsto the polymer surface. The halogenation solution can include any of avariety of suitable solutions. In some embodiments, the halogenationsolution includes hydrobromic acid, hydrochloric acid, phosphoroustribromide, phosphorous trichloride, hydroiodic acid, thionyl chloride,thionyl bromide, or combinations thereof. In some embodiments, thehalogenation solution can further include water.

In some embodiments, the halogenation step 120 is carried out by firstpreparing a solution including hydrobromic acid, hydrochloric acid,phosphorous tribromide, phosphorous trichloride, hydroiodic acid,thionyl chloride, or thionyl bromide. The solution can optionallyinclude water. The polymer surface can then be immersed in the solutionor, when the polymer surface is mounted in a module, the solution can beflowed through the module. Generally speaking, the polymer surface isthen exposed to the solution for a period of time during whichhalogenation of the surface of the polymer material occurs. After thedesired period of time has passed, the polymer surface is removed fromthe solution and rinsed.

Other suitable halogenation solutions include acyl halides containing aradical transferable group (e.g., bromoisobutyryl bromide). Thesehalogenation materials can be used to carry out the halogenation step120 in a similar or identical fashion to the method described in thepreceding paragraph.

Halogenation can also be performed using other suitable methods foradding halogen atoms to a polymer surface. In some embodiments, thehalogenation step can take place without the use of halogenationsolutions.

After the halogenation step is performed, a step 130 of performingsurface initiated atom-transfer radical-polymerization (ATRP) to form apolymer layer on the polymer surface is carried out. ATRP is a knowntechnique for conducting living polymerization in which one monomer isadded at a time to a growing polymer. In the method described herein,the growing polymer is a polymer layer being formed on the surface ofthe polymer material. Because of the slow, controlled growth, ATRP iswell suited for applications where tight control of monomer addition isdesired. In the method described herein, the slow, controlled growthallows for tight control of layer thickness.

The ATRP step 130 generally involves exposing the polymer surface to anATRP solution including various components necessary for conductingsurface initiated ATRP. One component of the ATRP solution is atransition metal element complexed with an organic ligand. Thetransition metal element contained in an organic ligand initiates theATRP by process by removing the halogen added in the halogenation step120 to thereby produce a chain extending from the surface of the polymermaterial and which includes a radical at the terminal end. This radicalis then attacked by a monomer molecule provided in the ATRP solution tothereby grow the chain by one unit. The growing chain bonded to thepolymer surface is the basis for a layer formed on the polymer surfacethat continues to grow so long as a radical continues to exist on thegrowing chain. The radical continues to attack additional monomer unitsin the ATRP solution to thereby grow the polymer layer. FIG. 2Cillustrates the ATRP process, wherein the halogen atom 220 (andoptionally the remains of the hydroxyl group) has been removed andmonomer units (M) 230 are added to the chain to begin formation of apolymer layer on the polymer surface 200.

Accordingly, step 130 generally involves exposing the polymer surface toan ATRP solution and allowing ATRP to take place such that a polymerlayer is formed on the polymer surface. Exposing the polymer surface toan ATRP solution can entail immersing the polymer surface in an ATRPsolution, or in the case of the polymer surface being mounted in amodule, flowing the ATRP solution through the module or filling themodule with the ATRP solution.

Generally speaking, the ATRP solution includes five main components: asolvent, a radically polymerizable monomer, a transition metal compound,a ligand, and a redox conjugate of the transition metal compound. Thespecific reagents chosen, the reagent concentrations, and the solventcan all be manipulated to allow for precise control of thepolymerization and, consequently, the thickness of the polymer layerformed on the polymer surface. Time in the ATRP solution can also beused to control layer thickness.

The solvent component of the ATRP solution can include any of a varietyof solvents suitable for use in ATRP processes. In some embodiments, thesolvent is toluene, 1,4-dioxane, xylene, anisole, dimethyl formamide,dimethyl sulfoxide, water, tetrahydrofuran, methanol, acetonitrile,chloroform, and combinations thereof. In some embodiments, the solventused is deoxygenated. In some embodiments, the solvent must include atleast water.

The radically polymerizable monomer component of the ATRP solution caninclude a variety of monomer types suitable for use in ATRP processes.In some embodiments, the radically polymerixable monomer is styrenes,methacrylates, methacrylamides, and acrylonitriles, derivatives thereof,and combinations thereof. Specific examples include, but are not limitedto, polyethylene glycol methacrylate and styrene species attached tochains of fluorocarbons.

The transition metal compound component of the ATRP solution caninclude, but is not limited to copper, ruthenium, iron, osmium,molybdenum, titanium, chromium, manganese, cobalt, rhodium, nickel,palladium, germanium, tin, bismuth and tellurium.

The ligand component of the ATRP solution is generally related to thetransition metal compound, as the two components generally form atransition metal compound bonded to the ligand so that during the ATRPprocess, the transition metal compound can work to remove the halogenatom and join it to the transition metal compound-ligand complex. Insome embodiments, the ligand is 2-2′ bipyridine,tris(2-aminoethyl)amine, 4,4′-Di-5-nonyl-2,2′-bipyridine,4,4′,4″-tris(5-nonyl)-2,2′:6′,2″-terpyridine,1,1,4,7,10,10-Hexamethyltriethylenetetramine, N,N-bis(2-pyridylmethyl)octadecylamine,N,N,N′,N′-tetra[(2-pyridyl)methyl]ethylenediamine,tris[(2-pyridyl)methyl]amine,tris(2-bis(3-(2-ethylhexoxy)-3-oxopropyl)aminoethyl)amine,Tris(2-bis(3-dodecoxy-3-oxopropyl)aminoethyl)amine,N,N,N′,N″,N″-pentamethyldiethylenetriamine, or diethylenetriamine.

The redox conjugate of the transition metal compound is also generallyselected based on the transition metal compound provided in the ATRPsolution. The redox conjugate is provided so that at least some of theinitially formed radicals in the ATRP process can be deactivated. Inother words, the redox conjugate is provided so that halogen atoms canbe transferred back to the polymer and terminate a portion of the activeradicals that would otherwise be the cites for further monomer addition.

Once the ATRP solution is prepared, the polymer surface is exposed tothe solution in order to form a polymer layer on the exposed surface viasurface initiated ATRP. As noted above, the polymer surface can beimmersed in a bath of the ATRP solution or the polymer surface can bemounted in module through which the ATRP solution is flowed or which isfilled with the ATRP solution. Generally speaking any area upon whichpolymer layer is to be formed should be exposed to the ATRP solution.The amount of time the polymer surface is exposed to the ATRP solutioncan impact the thickness of the layer formed, and can also be adjustedbased on the ratio of the components in the solution.

In some embodiments, ATRP reagents need to be regenerated in order tomaintain the ATRP process. The regeneration of ATRP reagents can becarried out using any suitable method. In some embodiments, ATRPreagents are regenerated by simply adding more of the desired reagent tothe ATRP solution being used in step 130. In other embodiments,electrodes can be incorporated into the method to regenerate some of thereagents. Other methods entail the use of ARGET ATRP or AGET ATRP. Inthese reaction configurations, the transition metal is regenerated fromthe higher oxidation state (where it inhibits the propagation of freeradicals) to the lower oxidation state (where it acts as an activatorfor radical polymerization). This regeneration is accomplished throughthe addition of reducing agents to the solution. Alternatively, thisregeneration may be accomplished through the addition of agentsproducing free radicals.

Termination of the ATRP step 130 can generally be carried out byremoving the polymer surface from the ATRP solution. In someembodiments, a cleaning step is carried out after the polymer surface isremoved from the ATRP solution in order to remove any residual reagentsfrom the surface of the material. Cleaning of the polymer surface havinga polymer layer formed thereon can be performed by, for example, rinsingthe material with pure water, acetone, methanol, or any suitable solvent(organic or otherwise) that is compatible with the material and capableof dissolving the desired residue.

The polymer layer formed on the polymer surface is generally anon-porous polymer layer. As used herein, non-porous generally refers toa negligible porosity, such as in the range of less than 1 nm. In someembodiments, the non-porous polymer layer still provides a desiredselectivity so that certain components may pass through the non-porouslayer. In some embodiments, the selectivity favors a similar selectivityto the material upon which the non-porous polymer layer is formed sothat the composite structure can be used beneficially for separationprocessing. For example, the underlying material and the non-porouspolymer layer may both have selectivities that favor the passage ofCO₂/CH₄ through the material so that the composite structure can be usedin natural gas sweetening.

As noted above, the thickness of the non-porous polymer layer can betightly controlled by varying a variety of factors. In some embodiments,the thickness of the non-porous polymer layer is in the range of from 75to 200 nm. Extremely thin layers can also be produced, such as less than75 nm or less than 5 nm. The non-porous polymer layer formed on thepolymer surface is also generally formed in a continuous and uniformfashion as a result of the ATRP process.

In some embodiments, the formation of the non-porous polymer layer onthe porous polymer material results in decreasing the size of the poresof the polymer material, and specifically the pores proximate thesurface of the polymer material on which the polymer layer is formed.The pore size is reduced by virtue of the polymer branches of thepolymer layer extending over the pores in the polymer material. In otherwords, the gap between pore walls in the polymer material is decreasedbecause of polymer chains that form on the surface of the polymermaterial and extend over the gap between pore walls.

In some embodiments, the atom-transfer radical-polymerization solutioncan include monomer units that are capable of crosslinking with similarmonomer units present in adjacent polymer chains. These monomer units,when incorporated into the individual polymer chains growing from thepolymer membrane surface during ATRP, can link together with similarmonomer units to thereby create links between adjacent polymer chains.This crosslinking activity can increase the strength of the polymerlayer formed on the polymer surface. The crosslinking can also changethe permeability of the layer and make the layer better suited forspecific applications. The crosslinking monomer units included in theATRP solution can generally include any monomer units capable ofcrosslinking. Exemplary crosslinking monomer units include poly(ethyleneglycol) methacrylate amine.

In some embodiments, monomers with multiple groups capable of radicalpolymerization may be used to achieve the effects of cross linking.Exemplary monomer units with multiple radically polymerizable functionalgroups include poly(ethylene glycol) dimethacrylate, ethoxylatedtrimethylolpropane triacrylate, 2,2-bis(prop-2-enoyloxymethyl)butylprop-2-enoate, and ethoxylated bisphenol A dimethacrylate,dipentaerythritol pentaacrylate.

In some embodiments, various steps can be taken prior to exposing thepolymer surface to any of the solutions described herein to remove, forexample, bubbles from the polymer surface and thereby ensure completesurface exposure. In some embodiments, the step includes sonicating theATRP solution after immersing the polymer surface in the solution.

Membrane Modules

As noted above, various steps of the method described above can beperformed by immersing the polymer surface in sequential baths. Varioussteps of the method can also be performed by using a closed module intowhich the polymer surface is mounted. In some embodiments, a combinationof both baths and modules is used. For example, step 110 can beperformed by immersing the material in a bath, while steps 120 and 130are performed in a module. In another example, steps 110 and 120 can beperformed by immersing the material in sequential baths, whileperforming step 130 with the material mounted in a module. In general,the mounting of the polymer surface in a module entails the use ofbundles of hollow polymer fibers, though the modules discussed below canalso be adapted for treating other forms of polymer material.

With reference to FIGS. 3-5, various modules suitable for use with themethods described herein are shown. The modules generally take the formof shell and tube-type vessels, wherein a first liquid can flow througha shell side of the reactor without flowing in the tube side of thereactor and a second liquid can flow through a tube side of the reactorwithout flowing in the shell side of the reactor. These configurationscan allow for selective formation of the polymer layer on the polymersurface, e.g., on only the exterior surfaces or only on the interiorsurfaces (in the case of hollow polymer fibers being mounted in themodule).

With reference now to FIG. 3, the module 300 can include a first end310, a second end 320, and a central region 330 located between thefirst end 310 and the second end 320. A bundle of hollow polymer fibers340 can be mounted in the central region 330. The bundle of hollowpolymer fibers 340 can also be potted with potting material 350 at eachend. The potting material 350 ensures that liquid entering at either thefirst end 310 or second end 320 can only flow through the interior ofthe hollow polymer fibers 340 and not between adjacent hollow polymerfibers 340 (i.e., the potting prevents the fluid from contacting theexterior surface of the hollow polymer fibers). The potting of thebundle of polymer fibers 340 can be accomplished by sealing the end ofthe bundle of the hollow polymer fibers 340 with a castable polymermaterial 350 such that the castable polymer 350 blocks both the interiorof the hollow polymer fibers 340 and the space between adjacent hollowpolymer fibers 340. The castable polymer 350 generally travels furtherup into the bundle 340 between adjacent fibers than within the hollowsof the fibers such that a slice can be made cross-sectionally throughthe bundle 340 at a point where the ends of the fibers are not blockedby castable polymer 350 but castable polymer 350 is still presentbetween adjacent fibers.

The module 300 further includes a tube side inlet port 311, a tube sideoutlet port 321, a shell side inlet port 312, and a shell side outletport 322. The tube side inlet port 311 and the tube side outlet port 321are in fluid communication with only the interiors of the hollow fibersin the potted bundle 340 mounted in the module such that fluid enteringthe module 300 via the tube side inlet port 311 can only travel throughthe interior of the hollow polymer fibers. The castable polymer material350 generally blocks the fluid entering from the tube side inlet 311from travelling between adjacent hollow polymer fibers. The shell sideinlet 312 and the shell side outlet 322 are in fluid communication withonly the exterior of the hollow fibers in the potted bundle 330 mountedin the module 300 such that fluid entering the module 300 via the shellside inlet port 312 can only travel around the exterior of the hollowpolymer fibers. In order to achieve this shell side fluid isolation, theshell side inlet port 312 and shell side outlet port 322 are generallylocated at the distal ends of the central region 330 and inside of thecasting material 360 as shown in FIG. 3.

When a polymer layer is to be formed on the exterior of the hollowpolymer fibers, the various solutions can be flowed through the shellside of the module 300. As noted above, any steps 110, 120 and 130 canbe performed this way. For example, if hydroxylation is to be carriedout on the exterior surfaces of the hollow polymer fibers, thehydroxylation solution is flowed through the shell side of the module300. Halogenation solution and ATRP solution can also be flowed in thismanner.

When a polymer layer is to be formed on the interior of the hollowpolymer fibers, the various solutions can be flowed through the tubeside of the module 300. As with the exterior coating, one or more ofsteps 110, 120, and 130 can be performed in this manner by flowingvarious solutions through the tubes side of the module 300.

In some embodiments, additional steps can be taken in order to protectthe potting material 350. The various solutions used can be corrosive tothe potting material and generally degrade the potting material overtime. Accordingly, in some embodiments a protective layer is added tothe inner and/or outer axial surfaces of the potting material. In thismanner, fluid flowing through the, e.g., tube side of the module 300will not directly contact the potting material adjacent the ends of thehollow. The protective layer can include, for example, fluorocarbonpolymers, silicone polymers, and polyurethanes.

In some embodiments, modules similar or identical to those describedabove are used as part of a circuit that recirculates the solution tocontinue carrying out the hydroxylation, halogenation, or ATRP reaction.With reference to FIG. 4, the circuit can include shell side tubing 410that fluidly connects the shell side outlet port 422 of the vessel 400to the shell side inlet port 412. Located between the inlet port 412 andthe outlet port 422 may be a pump 460 to help circulate the materialthrough the tubing 410. The tubing can also include a reservoir 470 thatincludes electrodes 471 used to regenerate various reagents, includingthe transition metal ions necessary to carry out the ATRP step. Theelectrodes can also be used to regenerate reagents used in thehydroxylation step and/or the halogenation step. The reservoir 470 maybe used to add, exchange, or replace reagents during the ATRP reaction.

FIG. 5 is similar to FIG. 4, but includes tube side tubing 510. The tubeside tubing 510 fluidly connects the tube side outlet port 521 of thevessel 500 to the tube side inlet port 511. Located between the inletport 511 and the outlet port 521 may be a pump 560 to help circulate thematerial through the tubing 510. The tubing 510 can also include areservoir 570 that includes electrodes 571 used to regenerate variousreagents, including the transition metal ions necessary to carry out theATRP step The electrodes 571 can also be used to regenerate reagentsused in the hydroxylation step and/or the halogenation step.

In some embodiments, a reel to reel system can be used to carry out themethods described herein in place of (or in conjunction with) themodules as described above. FIG. 6 provides an illustration of a reel toreel system 600 which provides a mechanism for passing the polymersurface through the various solutions. As shown, the system 600generally includes a bath vessel 610 which may be filled with any of thesolutions described above (e.g., hydroxylation solution, halogenationsolution, ATRP solution). The system 600 further includes reels 620,621. The first reel 621 includes wound material which is drawn off thewheel 621, passed through the bath vessel 610 containing the solution,and collected on the second reel 622. The system may include any numberof additional pulleys 623, 624, to guide the material through the bathvessel 610. The system 600 may also include a barrier layer 630 on topof the bath vessel 610 in order to create a closed vessel that preventsoxygen from contacting the solution in the bath vessel 610.

The configuration shown in FIG. 6 can be part of two or more sequentialsystems, with each system containing a different solution. In such anembodiment, the polymer material can begin on reel 621 and, instead ofbeing collected on a reel 622, can be passed on to a second systemcontaining a different solution. The collection reel 622 can bepositioned at the end of the last system. The system shown in FIG. 6 canalso be used in conjunction with a module. For example, a first reel toreel system can be used to carry out the hydroxylation step, followed bya second reel to reel system used to carry out the halogenation step.After the halogenation step, the material can be mounted in a moduleused for carrying out the ATRP step.

The reel to reel system shown in FIG. 6 can be used with a varietydifferent forms of material. For example, the material can be acontinuous piece of hollow polymer fiber thread that is subsequently cutinto discrete lengths. The reel to reel system is also adapted toprocess sheets of polymer material.

Applications

Natural Gas—Streams containing mixtures of various gas components may beseparated into two streams, each containing a higher percentage of oneof more of the components in comparison to the inlet stream usingmembrane units fabricated in accordance with the methods describedherein. Membrane modules manufactured using the methods described hereinmay be used in this capacity to remove carbon dioxide, hydrogen sulfide,and/or other components from natural gas streams. This is accomplisheddue to the higher permeability of the membrane to one gas component incomparison to another component. Use of the methods described herein toform membranes with thin polymer layers allows for previouslyunavailable polymeric materials to be used in the formation of membranesfor gas separation.

Additionally, modules fabricated using the methods described herein maybe linked in series to more efficiently separate a particular gasstream. These modules may be emplaced in series to increase the capacityof a membrane system. The modules may also be emplaced in arrangementsin parallel and cascade to separate a multicomponent gas stream.

Water Purification—Modules incorporating membranes fabricated using themethods described herein may be used to remove contaminants and desiredspecies from water. The polymer coating formed using the methodsdescribed herein can be designed to be permeable to water andimpermeable or relatively less permeable to the contaminants in thewater.

Blood oxygenation—Non-porous or relatively non-porous hollow fibermembranes formed using the methods described herein can be applicablefor blood oxygenation. The non-porous nature or relatively non-porousnature of the hollow fiber may prevent blood plasma leakage whileallowing sufficient blood gas exchange.

Lithium Ion battery separators—Porous separators are used in themanufacture of lithium ion batteries. These separators prevent certainions and molecular species from crossing while allowing others to moveuninhibited. The use of the methods described herein to from theseseparators can allow for the control of the pore size. This techniquecan be used to form desired surface coatings to enhance the performanceof these devices. The ability to form such a coating after the porousseparator is emplaced into the battery cell can improvemanufacturability and prevent defects.

Protein Purification—Porous membranes have been used to purifybiological molecules, particularly proteins. The ability to emplace awide range of coatings on the surface of membranes using methodsdescribed herein may increase the use of this technique. Methodsdescribed herein may also be potentially used to the control the poresize of these membranes, allowing for efficient size exclusionseparation.

Membrane Bioreactor—The ability to deposit tailored coatings on thesurface of membranes may be beneficial in the establishment of desiredbiological conditions. Pore size control using methods described hereinmay be beneficial in preventing certain biological species or solidparticles from crossing the membrane.

Membrane Distillation—The ability to apply a very thin coating ofdesired polymer using methods described herein may be beneficial for theproduction of membrane distillation devices, as the surface tension ofthe liquid and walls of the membrane pores is critical to the functionof these devices.

Dialysis—Blood contact with membranes during dialysis and oxygenationprocedures is unavoidable. The ability to deposit a tailored coating onthe surface of these membranes that prevents or inhibits coagulation andblood activation may significantly increase the performance of thesedevices. Such coating deposition may be carried out using methods asdescribed herein.

WORKING EXAMPLES Example 1

A composite hollow fiber membrane module was prepared by grouping 800polypropylene hollow fiber membranes with outer diameter of 300micrometers into a bundle. This bundle was then placed into a flask withtwo necks. The flask was then filled with one liter of deionized water.This water was then bubbled for a period of 20 minutes with argon toremove oxygen dissolved in the water. This flask was attached to acondenser and a rubber stopper was attached to the open neck. Throughthis rubber stopper, argon was continually bubbled. The flask was thenheated to 100° C. under reflux. At this point, 100 milligrams ofpotassium persulfate was added to the water and the solution waspermitted to react for 10 minutes. After this point, the fiber bundlewas removed, cleaned with deionized water, and then dried. The fiberbundle was then immersed into concentrated hydrobromic acid for 24hours. Afterward, the bundle was washed with deionized water and dried.

A solution containing 140 ml of deionized water and 60 ml ofpoly(ethylene glycol) methacrylate with approximately 520 repeatingunits of ethylene glycol was bubbled with argon to remove oxygen. 600 mgof copper (I) chloride, 160 mg of copper (II) chloride and 1.12 grams of2-2′ bipyridine were then added to the solution. This ATRP solution wasthen sonicated for a period of 10 minutes. The bundle of polypropylenehollow fibers were then immersed into this solution and allowed to reactfor 24 hours.

The bundle was then removed from the solution and washed thoroughly withwater and dried. It was then sealed into a stainless steel membranemodule using castable polyurethane. After hardening, the polyurethaneresin was severed with a sharp razor to reveal the open bores of thehollow fibers.

Example 2

A composite hollow fiber membrane module was prepared by grouping 800polypropylene hollow fiber membranes with outer diameter of 300micrometers into a bundle. This bundle was then potted into a stainlesssteel hollow module using a castable polyurethane resin.

200 ml of oxygen free, boiling deionized water were then mixed with 20mg of potassium persulfate and quickly poured into the hollow fibermembrane module so that the liquid contacted the outer surface of themembrane. This reaction was permitted for a period of 10 minutes.Afterward, the persulfate solution was removed and the module was rinsedwith a large amount of deionized water and dried.

200 ml of concentrated hydrobromic acid was poured into the membranemodule in such a manner as to contact all exterior surfaces of thehollow fiber membrane. This solution was allowed to react for 24 hours.Afterward, it was removed and the module, rinsed with deionized water,and dried.

A solution containing 140 ml of deionized water and 60 ml ofpoly(ethylene glycol) methacrylate with approximately 520 repeatingunits of ethylene glycol was bubbled with argon to remove oxygen. 600 mgof copper (I) chloride, 160 mg of copper (II) chloride and 1.12 grams of2-2′ bipyridine were then added to the solution. This ATRP solution wasthen sonicated for a period of 10 minutes. The solution was then placedin the membrane module and permitted to react for 24 hours. Afterward,the solution was removed, the module rinsed with water, and dried.

Example 3

A composite hollow fiber membrane module was prepared by grouping 800polypropylene hollow fiber membranes with outer diameter of 300micrometers into a bundle. This bundle was then placed into a flask withtwo necks. The flask was then filled with one liter of deionized water.This water was then bubbled for a period of 20 minutes with argon toremove oxygen dissolved in the water. This flask was attached to acondenser and a rubber stopper was attached to the open neck. Throughthis rubber stopper, argon was continually bubbled. The flask was thenheated to 100° C. under reflux. At this point 100 milligrams ofpotassium persulfate was added to the water and the solution waspermitted to react for 10 minutes. After this point, the fiber bundlewas removed, cleaned with deionized water, and then dried.

The fiber bundle was then placed into a dry, two necked flask equippedwith an air stop and a rubber stopper. The fibers were immersed in 230ml of tetrahydrofuran and 6 ml of triethyl amine. Argon was thensupplied through a needle, bubbling the solution. This solution was thencooled to 0° C.

In a separate flask, 5 ml of bromoisobutyryl bromide and 20 ml oftetrahydrofuran were combined and sealed. The contents of this secondflask were then transferred to the first flask anaerobically over thecourse of one hour. The solution containing the fiber was then permittedto rise to room temperature and react for 12 hours. Afterward, the fiberbundle was removed, washed with water, and dried.

The bundle was then removed from the solution and washed thoroughly withwater and dried. It was then sealed into a stainless steel membranemodule using castable polyurethane. After hardening, the polyurethaneresin was severed with a sharp razor to reveal the open bores of thehollow fibers

A solution containing 140 ml of deionized water and 60 ml ofpoly(ethylene glycol) methacrylate with approximately 520 repeatingunits of ethylene glycol was bubbled with argon to remove oxygen. 600 mgof copper (I) chloride, 160 mg of copper (II) chloride and 1.12 grams of2-2′ bipyridine were then added to the solution. This ATRP solution wasthen sonicated for a period of 10 minutes. This solution was then addedto the membrane module in such a fashion as to contact the outer surfaceof the fibers. This reaction was permitted for 24 hours. Afterward, themodule was rinsed with water and dried.

Example 4

A composite hollow fiber membrane module was prepared by grouping 800polymethyl pentene hollow fiber membranes with outer diameter of 200micrometers into a bundle. This bundle was then placed into a flask withtwo necks. The flask was then filled with one liter of deionized water.This water was then bubbled for a period of 20 minutes with argon toremove dissolved in the water. This flask was attached to a condenserand a rubber stopper was attached to the open neck. Through this rubberstopper, argon was continually bubbled. The flask was then heated to100° C. under reflux. At this point, 100 milligrams of potassiumpersulfate was added to the water and the solution was permitted toreact for 10 minutes. After this point, the fiber bundle was removed,cleaned with deionized water, and then dried. The fiber bundle was thenimmersed into concentrated hydrobromic acid for 24 hours. Afterward, thebundle was washed with deionized water and dried.

A solution containing 140 ml of deionized water and 60 ml ofpoly(ethylene glycol) methacrylate with approximately 520 repeatingunits of ethylene glycol was bubbled with argon to remove oxygen. 600 mgof copper (I) chloride, 160 mg of copper (II) chloride and 1.12 grams of2-2′ bipyridine were then added to the solution. This ATRP solution wasthen sonicated for a period of 10 minutes. The bundle of polymethylpentene hollow fibers were then immersed into this solution and allowedto react for 24 hours.

The bundle was then removed from the solution and washed thoroughly withwater and dried. It was then sealed into a stainless steel membranemodule using castable polyurethane. After hardening, the polyurethaneresin was severed with a sharp razor to reveal the open bores of thehollow fibers.

Example 5

A hollow fiber membrane module was constructed. A bundle of 500polyvinylidene difluoride hollow fiber membranes were immersed into a500 ml solution of water containing 20 g of lithium hydroxide. Thisreaction was permitted to occur for 24 hours. Afterward, the fibers wereremoved, rinsed with deionized water, and dried.

The bundle of fibers was then immersed into concentrated hydrobromicacid for a period of 24 hours. Afterward they were washed with deionizedwater and dried.

This fiber bundle was then potted into a stainless steel membranemodule. A solution containing 140 ml of deionized water and 60 ml ofpoly(ethylene glycol) dimethacrylate with approximately 800 repeatingunits of ethylene glycol was bubbled with argon to remove oxygen. 600 mgof copper (I) chloride, 160 mg of copper (II) chloride and 1.12 grams of2-2′ bipyridine were then added to the solution. This ATRP solution wasthen sonicated for a period of 10 minutes. This solution was then placedinto contact with the membrane module and permitted to react for 24hours. Afterward, the module was rinsed with water and dried. Thehardened resin was then severed with a sharp razor to expose the tubesof the membrane.

Example 6

A method similar to Example 2 was used to form bore coatings on theinterior surfaces of polypropylene hollow fiber membrane previouslypotted in a membrane module. The persulfate solution was pump throughthe bore of the hollow fiber membrane continuously for ten minutes.Afterward, water was flushed through the bore and the module was dried.Concentrated hydrobromic acid was then circulated through the bores ofthe fibers for 24 hours. Water was then flushed through the bores of thefibers. The ATRP solution was then circulated through the bores of thefibers for 24 hours. Afterward the tube side of the membranes wereflushed with water and the module was dried.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thescope of the invention. Accordingly, the invention is not limited exceptas by the appended claims.

Certain aspects of the technology described in the context of particularembodiments may be combined or eliminated in other embodiments. Furtherwhile advantages associated with certain embodiments of the technologyhave been described in the context of those embodiments, otherembodiments may also exhibit such advantages, and not all embodimentsneed necessarily exhibit such advantages to fall within the scope of thepresent disclosure. Accordingly, the present disclosure and associatedtechnology can encompass other embodiments not expressly shown ordescribed herein.

We claim:
 1. A method of forming a polymer film on a polymer fiber, themethod comprising the steps of: (a) cleaning the polymer fiber bysuccessively immersing the polymer fiber in one or more solvents; (b)hydroxylating the polymer fiber in a hydroxylation solution comprisingpotassium persulfate; (c) halogenating the polymer fiber; and (d)performing surface initiated atom-transfer radical-polymerization on thepolymer fiber, wherein the surface initiated atom-transfer radicalpolymerization forms a polymer film on the polymer fiber.
 2. The methodof claim 1, wherein the hydroxylation solution further comprises water.3. The method of claim 1, wherein the polymer fiber comprises a bundleof polymer fibers.
 4. The method of claim 3, further comprising the stepof potting the bundle of polymer fibers in a module after performingstep (a) and prior to performing step (b).
 5. The method of claim 4,wherein the bundle of polymer fibers comprises a bundle of hollowpolymer fibers and the bundle of hollow polymer fibers is potted in themodule such that only the exterior surface of the hollow polymer fibersis exposed to the hydroxylation solution.
 6. The method of claim 4,wherein the bundle of polymer fibers comprises a bundle of hollowpolymer fibers and the bundle of hollow polymer fibers is potted in themodule such that only the interior surface of the hollow polymer fibersis exposed to the hydroxylation solution.
 7. The method of claim 3,wherein: step (b) is performed by immersing the bundle of polymer fibersin a bath of the hydroxylation solution.
 8. The method of claim 3,wherein step (a) is performed prior to potting the bundle of polymerfibers in a module and steps (c) and (d) are performed after potting thebundle of polymer fibers in a module.
 9. The method of claim 3, whereinsteps (b) and (c) are performed prior to potting the bundle of polymerfibers in a module and step (d) is performed after potting the bundle ofpolymer fibers in a module.
 10. The method of claim 1, wherein the oneor more solvents comprises acetone, methanol, organic solvents, orcombinations thereof.
 11. A method of forming a polymer layer on abundle of hollow polymer fibers, the method comprising: providing abundle of hollow polymer fibers; cleaning the bundle of hollow polymerfibers by successively immersing the bundle of hollow polymer fibers inone or more solvents; potting the bundle of hollow polymer fibers in amodule; halogenating the bundle of hollow polymer fibers with ahalogenation solution; and performing surface initiated atom-transferradical-polymerization on the bundle of hollow polymer fibers, whereinthe surface initiated atom-transfer radical polymerization forms apolymer film on the hollow polymer fibers; wherein the halogenationsolution comprises hydrobromic acid, hydrochloric acid, phosphoroustribromide, phosphorous trichloride, hydroiodic acid, thionyl chloride,thionyl bromide, or combinations thereof.
 12. The method of claim 11,wherein the method further comprises the step of hydroxylating thebundle of polymer fibers prior to performing surface initiatedatom-transfer radical-polymerization.
 13. The method of claim 12,wherein hydroxylating is carried out using a hydroxylation solution. 14.The method of claim 13, wherein the hydroxylation solution comprisespotassium persulfate, ammonium persulfate, lithium hydroxide, orcombinations thereof.
 15. The method of claim 11, wherein the one ormore solvents comprises acetone, methanol, organic solvents, orcombinations thereof.